head to head
Neither is simply harder - they are hard in different places. FIFA 21 demands more variance; Super Smash Bros. Melee demands more execution.

Execution · Competitive PvP
Football sim testing execution and opponent reading under a studied tactical meta.

Execution · Competitive PvP
A platform fighter with one of the highest execution ceilings in any competitive game, layered on top of deep human reads.
FIFA 21 leans Execution (Competitive PvP); Super Smash Bros. Melee leans Execution (Competitive PvP).
Super Smash Bros. Melee leans clearly harder on execution.
FIFA 21
Skill moves and finishing mechanics demand practiced hands
Super Smash Bros. Melee
Wavedashing, L-cancels, and tech-chasing set an extreme physical ceiling
Super Smash Bros. Melee leans clearly harder on reads.
FIFA 21
Outguessing a human's defensive shape and attacking patterns is central
Super Smash Bros. Melee
Reading DI choices and opponent adaptive habits is constant and non-convergent
FIFA 21 leans clearly harder on variance.
FIFA 21
Deflections, finishing RNG and pack luck colour outcomes
Super Smash Bros. Melee
Stage hazard RNG and spawn positions add marginal chaos
Super Smash Bros. Melee leans a little harder on optimization, though the gap is small.
FIFA 21
Meta formations, custom tactics and squad building are studied deeply
Super Smash Bros. Melee
Character matchup tiers and optimal movement routing are deeply studied
A matching score does not mean a matching test. Two games can both sit at 10 on execution and demand completely different things of you - which is why the reasoning above matters more than the number beside it.